As devices shrink, it is becoming more difficult to accurately overlay layers to meet device requirements. Overlay issues can arise from the shifting of layers, which can cause defects in the device. For example, the shifted layers will result in misaligned features throughout the layers. To adjust for these issues, measurements of the shift are desired to determine an amount of shift so that appropriate correction can occur.
Issues can also arise from the measuring of the shift, itself. As an example, measurement noise can result from discrepancies in the measurement technique being implemented. For example, measurement noise can occur because of discrepancies in an optical overlay measurement technique, which measures the shift by looking from a top layer down through to a bottom layer, i.e., in a y-axis. Process variation during fabrication can also cause measurement noise, resulting in further inaccurate measurements. In this way, measurement noise can significantly affect correction of the shift and have overlay errors.